An electrophotographic copying machine, or the like, is provided with a fixing device for fixing a toner image transferred onto the recording material by heat and pressure.
As a fixing device, various types are known to raise a temperature at a high speed. Those fixing devices in which a fixing roller is made thinner and smaller in diameter, those fixing devices in which a heating member is pressed against a rotating member, formed of a resin film, from an inside thereof, those fixing devices in which a thin metal rotating member is heated by induction heating, and the like, are known. All of these fixing devices are designed to reduce a heat capacity of the rotatable member, which is a heating member, and to heat the rotatable member with a heat source with a high heating efficiency.
In Japanese Patent Document No. 2004-279702, in order to prevent the production of a crease in an envelope, the pressing force per unit area of a first pressing roller and a second pressing roller to the heating roller is changed between a normal pressure mode and an envelope pressure mode. In the envelope pressure mode, the pressing force per unit area to the heating roller of the first pressing roller and the pressing force per unit area to the heating roller of the second pressing roller are reduced to prevent production of an envelope crease.
In the apparatus described in Japanese Patent Document No. 2013-225039, in order to prevent production of an envelope crease, the shape of a fixing nip portion N is switched from a nip mode in which a flat portion and a curved portion are in contact, to a nip mode in which only the flat portion contacts.
In the structures described above, however, in order to improve the prevention of the envelope crease, it is necessary to lower the pressing force per unit area more than necessary, and there is room for improvement.